Roof Framing and Exterior Finish
Roof geometry
Roof slope is rise over run, usually written as inches of rise per 12 in of run (a 6/12 roof rises 6 in for every foot of horizontal run). Pitch is the same relationship expressed as a fraction. The run of a rafter is half the building span, not the full span - a common trap.
Worked example: a 5/12 gable roof on a 20 ft wide building. Run = 10 ft = 120 in. Rise = 5 × 10 = 50 in. Rafter length = √(120² + 50²) = 130 in, or 10 ft 10 in, before adding the overhang.
Know the rafter family - common, hip, valley, jack, cripple - plus the ridge board, collar ties, rafter ties, birdsmouth seat cut, plumb cut, and lookouts. Roof sheathing is laid with the long edge supported and properly gapped, and underlayment follows the manufacturer's slope requirements.
Exterior finish
Water management is the theme: weather-resistant barrier over sheathing, flashing at valleys, walls, and penetrations, drip edge at eaves and rakes, and siding held 6–8 in above grade. Lap siding courses are kept level and lapped correctly, and corner boards and trim are installed so water sheds outward.
Traps: using full span instead of half span; confusing pitch with slope percentage; forgetting overhang when cutting rafters; skipping head flashing above windows.